Because of many advantages including impact resistance, light weight, and workability, organic resin materials are used in a wide variety of applications. Efforts are currently made to take more advantage of these properties. One such approach is to apply molded organic resins having enhanced surface hardness and abrasion resistance to the windows in various vehicles. In the glazing application, a high level of abrasion resistance and outdoor weather resistance comparable to glass are required. In the case of automobiles, for example, a high level of abrasion resistance is required in order to prevent the windshield from marring upon wiper operation and to prevent side windows from marring upon winding up-and-down operation. Potential service in a very high temperature or humidity environment must also be taken into account.
In the prior art, substrates of organic resins or plastics are surface coated with various coating compositions to form surface protective films for the purpose of imparting high hardness and mar resistance. For instance, compositions comprising hydrolyzates or partial hydrolyzates of hydrolyzable organosilanes and optionally, colloidal silica are known.
For instance, JP-A S51-2736, JP-A S53-130732 and JP-A S63-168470 disclose coating compositions comprising an organoalkoxysilane, a hydrolyzate and/or partial hydrolyzate of the organoalkoxysilane, and colloidal silica, wherein the alkoxy group is converted into silanol in the presence of excess water. However, these coatings resulting from wet coating systems suffer from problems of low hardness and poor mar resistance as compared with glass or the object to be replaced.
However, several problems must be solved before coating films can withstand sunlight and weather over a long time. The wet or dry coating layers having mar resistance lack an ability to cut UV, and a phenomenon develops that a resin substrate, a primer layer for imparting substrate adhesion or an interface therebetween can be degraded or discolored by UV exposure. Several techniques are proposed to prevent such a phenomenon, including addition of UV absorber to the primer layer, and incorporation via chemical bonds of UV absorptive organic substituent groups into the organic resin of which the primer layer is formed. The UV absorptive organic substituent groups and UV absorbers refer to benzophenone, benzotriazole, triazine and similar substituent groups, and organic compounds containing the same. See JP-A H04-106161, JP 3102696, JP-A 2001-47574, and JP 3841141.
The above technique for cutting off UV is by incorporating an organic UV absorber into a primer layer. Since the primer layer in itself has the main purpose of improving the adhesion between the underlying substrate and a silicone layer, an extra amount of UV absorber loaded gives rise to problems such as losses of adhesion and transparency. It is demonstrated in a long-term outdoor exposure test and accelerated weathering test that the UV cut by the primer layer alone is insufficient for preventing degradation and discoloration of organic resin substrates.
One approach taken for compensating for such drawbacks was to add organic UV absorbers to silicone layers as well. However, simply adding such compounds to coating compositions results in a coating lacking durability. That is, the coating fails to sustain the desired UV absorbing property due to bleeding and drainage of UV absorber from the surface during long-term weather exposure. Then organic UV absorbers were developed which are silyl-modified so as to be chemically bondable with siloxane compounds, the main component of the coating layer. See JP-B S61-54800, JP-B H03-14862, JP-B H03-62177, and JP-A H07-278525. This measure improves retentivity since the UV absorber is strongly bound to the siloxane matrix. On the other hand, these coating layers become substantially poor in mar resistance that is essentially desired, or develop noticeable microcracks due to a lowering of flexibility. As discussed above, the organic UV absorbers have the essential drawback that the hardness of silicone film becomes lower as the amount of UV absorber added is increased to enhance weather resistance.
In another attempt, metal oxide nanoparticles having UV shielding property are added to coating compositions so that the compositions may maintain hardness and mar resistance. Known examples are titanium oxide nanoparticles of anatase type (JP-A 2004-238418) and titanium oxide nanoparticles of rutile type (JP 2783417, JP-A H11-310755, JP-A 2000-204301). These coating compositions form UV-shielding coatings which maintain visible light transmitting and mar resistant properties. However, titanium oxide nanoparticles have a photocatalytic activity which cannot be fully suppressed even when they are surface coated with silicon compounds. Additionally, the coatings have insufficient weather resistance in that cracks develop in a relatively early stage in an accelerated weathering test.
It is also known to use zinc oxide nanoparticles as the metal oxide nanoparticles having UV shielding property (see JP-A H11-209695, JP 3347097, and JP-A 2002-60687). In general, the zinc oxide nanoparticles have somewhat poorer UV shielding property than the titanium oxide nanoparticles and accordingly lower photocatalytic activity. However, on account of residual photocatalytic activity, a coating loaded with zinc oxide nanoparticles can not avoid a phenomenon that the coating develops cracks or peels in a weathering test.
JP 3509749 and JP-A 2002-87817 disclose an attempt to suppress photocatalytic activity by coating surfaces of zinc oxide nanoparticles with another oxide. A coating loaded with surface-coated zinc oxide nanoparticles has a longer lifetime in a weathering test than bare zinc oxide nanoparticles. However, the coating is still insufficient as outdoor UV shielding members partly because cracks develop in a long-term weathering test.
In general, visible light transparency is one of important properties of coating compositions for forming weather resistant surface protective coatings. If metal oxide nanoparticles are used as the UV shielding agent, visible light transparency is substantially impaired depending on an average particle size and a tendency to agglomerate. JP-A H11-278838 discloses that when zinc oxide nanoparticles are prepared by a specific method, a dispersion thereof has a smaller particle size and is unsusceptible to agglomeration. A coating composition having this zinc oxide nanoparticle dispersion compounded therein would be highly transparent to visible light although this is not described in Examples.
It is also proposed to deposit oxide thin films such as silicon dioxide on various cured resin layers in order to achieve the high level of abrasion and mar resistance required for automotive windows. See Patent Documents US2005/0202263A1, JP-A 2009540124T, JP-A 2009502569T, U.S. Pat. No. 7,163,749 B2, U.S. Pat. No. 7,056,584 B2, U.S. Pat. No. 6,376,064 B1, and U.S. Pat. No. 4,927,704 A. It is confirmed that these coatings resulting from dry coating systems are significantly improved in mar resistance over the aforementioned wet coatings. Additionally, the dry coatings impart protective, barrier properties onto the underlying coatings, enhancing the weatherability and increasing the lifetime of the coated window.
Expanding Thermal Plasma (ETP) processes have been used to deposit the dry coatings at high deposition rates, such as JP-A 2008504652T and U.S. Pat. No. 7,282,244 B2. Such plasma processes are suitable for coating highly abrasion and mar resistant coatings in large scale and on large area parts, such as described in JP-A 2008504652T, U.S. Pat. No. 7,282,244 B2, US 2008/0286492 A1, US 2008/0160205 A1, US 20080160197 A1, JP-A 2008509283T, U.S. Pat. No. 7,645,492 B2, U.S. Pat. No. 7,390,573 B2, U.S. Pat. No. 7,595,097 B2, U.S. Pat. No. 6,948,448 B2, U.S. Pat. No. 6,681,716 B2, U.S. Pat. No. 6,641,673 B2, JP-A 11071681, U.S. Pat. No. 6,213,049 B1.
As discussed above, a number of attempts have been made to improve the weather resistance, mar resistance and other properties of wet and dry coating films. However, there is not available a laminate having a coating system which exhibits visible light transparency and UV shielding property, and sufficient weather resistance and durability to withstand prolonged outdoor exposure while maintaining a very high level of mar resistance comparable to glass.